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Astro-Physics 1100GTO German Equatorial Mount Question

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#1 extremeastrobabe

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Posted 25 August 2019 - 01:30 AM

Hello! 

 

I am very new to telescopes and have access to a wonderful telescope through my astronomy club. Our club is relatively new and thus there is no one with a lot of experience with our mount/telescope to guide us and teach us. 

 

We have a Celestron Edge HD 11" telescope that goes on this mount and we have noticed an issue we are having that I am trying to figure out is normal or if there is something off, such as orthogonality. 

 

The telescope and mount are not permanent, and are always broken down and put up every time that we use it. We set it up and polar align it with RAPAS. After that we slew to a bright star and RE-CAL on it. Now if it stays on one side of the meridian, it is fine. It will have no problem locating objects and centering on them. However when it does a meridian flip, it will then be off by about .5 degrees. Without doing another RE-CAL on that side, if we slew back then it centers again no problem. This seems odd to me. How can it be fine on one side, off on the other, and then go back to the same side and be fine again? Doesn't matter if you slew east and RE-CAL or if you start with the west and RE-CAL. As soon as it crosses the meridian it is off by .5 degrees. 

 

Is it normal for the scope to be fine on one side, say the west, and then when slewing to the east and crossing the meridian to be off by a little bit? I don't believe that we have tested it by slewing to the west and doing RE-CAL and then slewing east and doing RE-CAL and then going back west then east again to see if it still centers on both sides. Or we might have but I cannot remember. I also am not sure I am fully understanding what RE-CAL is. When it re-calibrates, does it re-calibrate on one object and then if you re-calibrate on another it forgets that object? Or if you re-calibrate on more than one object, it remembers all of them and provides a more accurate calibration? 

 

We only use a keypad to control the mount right now. So any help for using the keypad only would be awesome! 

 

And any help would be greatly appreciated before we start trying to address possible orthogonality issues. Thank you in advance!!! smile.gif


Edited by extremeastrobabe, 25 August 2019 - 01:42 AM.


#2 DiscoDuck

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Posted 25 August 2019 - 02:59 AM

I am no expert, but it does sound very much like orthogonality error in the scope to me. Most mounts estimate and correct for this when you do a 3 star alignment. But Astro-Physics so far have not supported that (though I understand from discussions on the forum that may change with the next version of their controller).

 

One excellent utility for measuring cone/orthogonality error is ConeSharp

 https://www.sharpcap.co.uk/conesharp

(from the writer of SharpCap). It's free and very quick to use and gives an estimate of orthogonality error in arcminutes. You can then shim/adjust the scope accordingly.

 

P.S. But if, as you say, you're 0.5 degrees out after flipping, it's probable your error is 0.25 degrees. So ConeSharp may not be necessary, other than to tell you which direction to adjust the scope in!


Edited by DiscoDuck, 25 August 2019 - 03:01 AM.

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#3 WadeH237

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Posted 25 August 2019 - 10:18 AM

The behavior you describe is expected for a scope that is non-orthogonal.

 

Astro-Physics is working on a future revision to the CP4 controller that will allow for software compensation for orthogonality error, but I would not expect it soon.  They are busy getting the new Mach 2 mount ready for release, so the CP4 is probably not their priority at the moment.

 

In the meantime, you have two options, assuming that you are using the keypad.  The first is to do what you are doing, and just recal in the area where you will be observing.  This is what I do with my C14 on an AP1100.  If you want, you can also fix the orthogonality by using shims between the dovetail and the telescope.  Astro-Physics has instructions for doing this.  I would suggest either reaching out to them on the AP-GTO user group.  Or better yet, give them a call.  George would probably be happy to help you out with this.

 

If you are going to use the mount and scope for imaging, then you could use the Pro version of APCC to control the mount from a computer.  It supports building a tracking and guiding model that will correct for many factors, including orthogonality.



#4 rgsalinger

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Posted 25 August 2019 - 11:09 AM

If the system is broken down each night I would not expect that an APPM model created with APPC would still be useable on subsequent nights. I know that you can verify an existing model so that might be the trick. However, just building a small - 12 point model - each night will get your targets near the center even with a C11 at full focal length. When I started using a CDK14, I did exactly that - small model - just to get the target into the FOV and it worked quite well for that purpose.

Rgrds-Ross



#5 extremeastrobabe

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Posted 25 August 2019 - 03:18 PM

Thank you all so much for your replies!

 

So, I just want to make sure I am understanding but is orthogonality an issue with the telescope or the mount? Where does it come from? I have read their page about it in the manual but I'm not sure I understand it completely. If it's because there's something that's not perfectly aligned together, does it really cause that much of an issue? 

 

I also don't understand if it's shifting a bit when slewing across the meridian why is it able to center back with no problems when returning to the other side. 

 

I have been in contact with George at Astro-Physics but I was also hoping to reach out to others who might have possibly had the same problem :)

 

It seems, too, that it might be a bit easier to correct for if we are using a computer, which we are not. We are still so very new, but definitely heading that direction! 



#6 WadeH237

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Posted 25 August 2019 - 07:30 PM

Orthogonality can be an issue with the mount, but I would be very surprised to see it on an Astro-Physics mount or any of its direct competitors.  In that category, I would consider it a defect that should be addressed by the manufacturer.  Orthogonality is a very common issue with telescopes.  I would go so far as to say that it would be very rare to find a telescope without at least some orthogonality error. 

 

To be clear, here is what it actually is (assuming a telescope issue):

 

A system with no orthogonality error has the mount's RA axis perfectly parallel to the telescope's optical axis (ie. which direction the scope is actually pointing).  Now picture the mount in what most people consider the home position.  That is with the counterweight shaft pointed down and the scope pointed north (or south, if you are in the southern hemisphere).  It is possible for the scope to point a little bit to the left or right of the RA axis, and this is actually ok.  When you sync or recalibrate the mount, it will adjust the declination offset just a bit to compensate.

 

Now consider that the telescope is pointing just a little bit above or just a little bit below the RA axis.  This is a problem.  First off, it means that the telescope could never center the celestial pole in the telescope (there is no axis on the mount that could move telescope up or down in that direction).  If the telescope is pointed parallel to the RA axis and focused at infinity, rotating the RA axis would rotate the field of view in the telescope, but the center of the field would not change.  With orthogonality error, when the RA axis rotates, the telescope's line of site transcribes a cone, with the tip a the mount and the circular base projected ever wider in the sky.  You can see, then, that as the RA axis rotates, the image in the scope rotates and shifts.  If the scope in the home position were pointed a bit above the RA axis, when the RA axis is turned 180 degrees, it would point the same amount low.

 

The meridian flip is actually a 180 degree rotation of the RA axis, and that same change in the direction the scope is pointed happens.

 

The mechanical fix is to shim either the front or back of the dovetail, depending on whether the scope is pointed high or low in the home position.

 

I hope that makes sense.


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#7 noisejammer

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Posted 02 September 2019 - 11:33 PM

 Like Wade, I'm really sceptical that the problem is with the mount but it could easily be in the scope.

 

When shimming the scope, you could use folded aluminum foil to take up half the angle that you get from opposite sides of the pole.As an example, if you want to correct 0.25 deg offset and the scope's mounting points are 18" apart, you need to make a shim that's 18" * 0.25/57.3 or about 0.078" thick.



#8 JoeR

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Posted 03 September 2019 - 08:02 AM

I use the 1100GTO with the Keypad and this is normal. With only the one star calibration available it will always need re-calibration every time you flip the meridian. I shimmed my C14 OTA's dovetail on the front with stainless steel washers and that reduced the error but there is still some, enough to put the object out of the FOV, and that is inevitable. It's especially inevitable with an OTA that has a moving primary mirror. Differential flexure will create a certain amount of cone error that can't be shimmed out.



#9 555aaa

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Posted 03 September 2019 - 10:07 AM

To be clear, this isn't a problem with the mount, it's that the telescope optical axis is not perfectly aligned to the dovetail which is normal. In addition to Wade's suggestion above, you might consider PRISM which includes mount modeling. Run a small model on both sides of the pier. The model terms will tell you what the errors are. If there is a lot of mirror flop then the model will have a lot of scatter. I think you can download PRISM on a trial basis but APCC is probably the best solution.
ETA I'm not sure if PRISM will let you build a model using only a crosshairs eyepiece but there is other software that will. In theory you can gather the pointing info with pencil and paper and then type it into the modeling tool e.g. Point. You can also check cone error in the daytime.

Edited by 555aaa, 03 September 2019 - 10:50 AM.


#10 Christopher Erickson

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Posted 03 September 2019 - 04:41 PM

Let me add that a scope MUST be carefully collimated before orthogonality can be accurately evaluated and corrected. This is especially true in SCT's and Newts and not so much in refractors. Collimation moves the optical axis around inside of the optical tube.

 

Also make sure everything in the optical train is tight and slop/flex free, as much as is possible.

 

Some SCT's and Newts have a lot of component flexure and/or mirror flop. In this case a precise orthogonality might not be possible. However in most cases ortho can be refined enough to make life easier.

 

And there are ways to live with ortho error fairly easily. One way is when you flip the meridian, do a GOTO a bright star and then "re-calibrate" on that side of the meridian should get everything back in the eyepiece or on the chip until the next meridian flip, where you will "re-calibrate" again. Another trick that will help with meridian flips is using your lowest-power eyepiece to find the target after a meridian flip, and also knowing that a majority of the error will be in the E-W directions, not the N-S directions, which should help narrow down the search area quite a bit.


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